Diode string triggered silicon controlled rectifiers (SCR) are becoming dominant electrostatic discharge (ESD) devices for high frequency I/Os, as well as becoming ever more popular for use on small voltage island power supplies (low voltage applications). However, such devices have contradictory needs. For example, ESD devices require low trigger current; whereas, higher SCR trigger current is required for normal operations of the SCR to avoid mistriggering but fast turn-on during ESD events.
FIG. 1a shows a schematic of a conventional diode string triggered silicon controlled rectifier (DTSCR) based electrostatic discharge (ESD) device. More specifically, FIG. 1a shows a schematic of a three string diode dual well ESD DTSCR. In this representation, three diodes are in series, with a fourth diode in the body of the SCR. The SCR also includes resistor Rpw and resistor Rnw.
As should be understood by those of skill in the art, the three diode string determines the trigger point voltage of the DTSCR. That is, the diode string controls the voltage to turn on the SCR. For example, in operation, the diode string is triggered (Itrig) to control the voltage in order to turn on the SCR. Also, in operation, the ESD DTSCR discharges ESD current to ground during a positive mode ESD event.
FIG. 1b shows a layout view of a conventional DTSCR based electrostatic discharge (ESD) device of FIG. 1a. More specifically, FIG. 1b shows a cross sectional view of the SCR with cross coupled bipolar transistors which is integrated with trigger diodes to form the. In FIG. 1b, the SCR includes a P+ diffusion and N+ diffusion in a P-well (PNP) and a P+ diffusion and N+ diffusion (NPN) in an N-well. The resistor Rpw is provided in the P-well and the resistor Rnw is in the N-well. As should be well understood, the magnitude of the current (Itrig) is proportional to the substrate resistance.
FIG. 2 shows a graph of voltage vs. current for the conventional DTSCR based electrostatic discharge (ESD) device of FIGS. 1a and 1b. As shown in the graph, at Von, the four diodes of FIG. 1a are turned on, and the SCR starts to conduct current. At Vtrig, Itrig, sufficient current is in the diodes in order to turn the SCR in an on state. That is, the diode string is triggered (Itrig) to turn on the SCR. At Vh, Ih, current is no longer conducting through the diodes. Instead, at this stage, the current is conducting through the SCR to ground though the PNPN ground terminal.
As noted above, the magnitude of the current, Itrig, is proportional to the substrate resistance, Rpw. That is, Itrig is controlled by Rpw. As such, if Itrig is desired to be low, it is necessary to have an increased substrate resistance (Rpw) high. Also, if Itrig is desired to be high to trigger the SCR, it is necessary to have lower Rpw which will require larger external diodes. The larger external diodes, though, consumes area and capacitance. For example, referring to FIG. 1b, to increase Rpw and lower the Itrig, it is necessary to move the Rpw to the left of FIG. 1b, thus increasing the total area of the structure which further wastes valuable substrate real estate.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.